Is there an addon for todo management?
You can use Generic Note Add-on which allows to create some kind of sticky note in the Compositor. In this case I'd suggest re-using a text file from the Text Editor:





In addition there is a nice Comment Node Add-on by @CoDEmanX, which allows creating a list:





Since Blender 2.74, you can use the built-in Frame Node to display text blocks in the Node Editor: 



Further information: Is there a way to make comments in the node editor?